Known are ventilators or respirators for the mechanical, artificial respiration for all forms of an oxygen deficiency state. They are, inter alia, applied for the long-time respiration. (Roche Medical Dictionary, 4th Edition, edited by Hoffmann-La Roche AG and Urban & Fischer, Urban & Fischer, Munich, Stuttgart, Jena, Lübeck, Ulm).
A particular form of ventilator, so-called CPAP apparatus, are used to avoid obstructive respiratory disturbances during sleep.
Obstructive respiratory disturbances result in apneas which make the sleeping person wake up. Frequent apneas prevent the sleeping person from falling into the relaxing deep sleep. People suffering from apneas during sleep therefore are tired during the day, which may result in social problems at work and, in the worst case, in deadly accidents, for example with professional motorcar drivers.
The CPAP (continuous positive airway pressure) therapy was developed for the treatment of apneas and is described in Chest. Volume No. 110, pages 1077-1088, October 1996 and in Sleep, Volume No. 19, pages 184-188. A CPAP-apparatus generates a positive overpressure up to approximately 30 mbar by means of a fan and administers said positive pressure, preferably via a humidifier, via a ventilator tube and a nose or face mask to the respiratory tract of the patient.
This positive pressure is to ensure that the upper respiratory tract remains fully opened during the whole night, so that no apneas will occur (DE 198 49 571 A1). The required positive pressure is also called therapeutic pressure pt and depends, inter alia, on the sleeping stage and the position of the body of the sleeping person.
FIG. 1 shows a CPAP-apparatus 1 during therapeutic use. The CPAP-apparatus 1 comprises a housing 4, a ventilator tube 9, and a nose or face mask 18. The housing 4 includes a fan 8 also being called a compressor, blower, ventilator or turbine. A pressure sensor 11 for measuring the positive pressure, generated by the fan, with respect to the ambient pressure is provided in the proximity of the ventilator tube connection inside the housing. The measured positive pressure will hereinafter be called actual pressure. The air conveyed by the fan 8 is supplied via a ventilator tube 9 to the face mask 18 worn by the patient 19 himself. In the face mask 18, or in the proximity thereof, an expiration opening 2 is provided, through which a permanent air flow occurs from the ventilator tube into the ambiance. This air flow ensures that the air expired by the patient is carried off into the ambiance and prevents the accumulation of CO2 in the ventilator tube 9. A microcontroller 5 controls the number of revolutions of the fan such that the actual pressure measured by the pressure sensor 11 corresponds to a target pressure.
With all known CPAP-apparatus, and according to the patent literature, so far two methods are used so as to determine the pressure in the face mask of CPAP-apparatus.
According to one thereof a pressure sensor detects the pressure of the air conveyed to the patient directly in the apparatus. In the factory, for a frequently used combination of ventilator tube and mask the pressure difference is measured at a medium volume flow. This differential value is deducted from the pressure measured in the apparatus, and the result is interpreted as mask pressure. Errors in the pressure adjustment are inevitable because the air flow permanently fluctuates due to the respiration so that the pressure loss in the ventilator tube varies. It is thereby particularly annoying for the patient that the pressure in the face or nose mask is lower during inspiration as compared to expiration. Particularly with this form of pressure control the patient therefore has the feeling that he has to respire against a resistance.
According to other methods, one of which is described in WO 00/66207, the detection of the pressure is effected in the proximity of the end of the ventilator tube in front of the mask or in the mask. The pressure detection is thereby very exact. The pressure control also compensates leakages etc. The pressure measurement itself may be accomplished with a pressure sensor the electrical connecting leads of which have to be guided with the ventilator tube to the ventilator. In particular with apparatus from MAP is a separate thin tube guided from the measuring point to a pressure sensor disposed in a CPAP-apparatus. A drawback of these variants resides in that special tubes are necessary so as to bring the connecting leads or the tube, respectively, back into the CPAP-apparatus. These custom-made products are more expensive than ordinary ventilator tubes. Moreover, their use with other CPAP-apparatus is possible only to a limited extent. Finally, the cleaning is more complicated.